The present invention relates generally to optical fibers, more specifically to amplifier fibers which contain an absorbing outer glass layer to minimize transmission of cladding modes.
A typical geometry for an amplifier fiber 10 is illustrated in FIG. 1, where the fiber contains a circular core 12 at the center of the fiber, a cylindrical clad 14 surrounding the core, a primary or inner polymeric coating 16 which surrounds the clad, and a secondary or outer polymeric coating 18 which surrounds the primary coating. Typically, the refractive indices of the various materials used to prepare the optical fiber will be as follows: a relatively high refractive index for the core 12, a refractive index for the clad 14 which is lower than that of the core, and a refractive index for the primary coating 16 which approximates or is greater than that of the clad. Generally, the refractive index of the secondary coating 18 can be independent of the indices of the other materials.
The purpose of the higher core index is to cause light injected into the fiber to propagate along the core rather than through the clad. The refractive index of the primary coating approximates the refractive index of the clad at the wavelength of interest. If the refractive index of the primary coating is less than that of the clad, then the clad itself can act as a waveguide, much like a huge core within a plastic clad. When light escapes from the core and propagates down the clad (rather than the core), it is referred to as a cladding mode. The phenomenon results in detrimental interference and distortion of the signal in the core. Thus, a need exists for a glass fiber construction which is capable of reducing the propagation of cladding modes.
In telecommunication applications, the primary coating is typically highly absorbing in the infrared. This is largely due to the predominance of Cxe2x80x94H bonds, which have high vibrational frequencies and the overtones of which produce near opacity over very short path lengths. As a result, an index-matched polymer layer outside the clad will extinguish cladding modes as they propagate along the length of the fiber. There are cases, however, in which it is difficult to design a deployable polymer coating to index match a glass. For example, when clad indices exceed 1.6, then it becomes necessary to incorporate halogenated or aromatic hydrocarbons, which can pose health risks and may be water sensitive. At still higher indices,  greater than 1.8, it may be very difficult to prepare any plastic polymer with an appropriate refractive index, much less one that can survive service conditions. Likewise, a need exists for an optical amplifier fiber which utilizes a glass fiber construction that can achieve a reduction in cladding mode propagation while using conventional primary and secondary coatings.
The present invention is directed to overcoming these deficiencies in the art.
One aspect of the present invention relates to an amplifier fiber. The fiber includes a glass core and a glass cladding layer surrounding said glass core. The glass cladding layer has a refractive index which is less than a refractive index of the glass core. The fiber also includes a glass overclad layer surrounding the cladding layer. The overclad layer has a refractive index which is greater than the refractive index of the cladding layer. Alternatively, the overclad layer can be doped with an absorber for stripping a mode of light propagating in the cladding layer of the fiber from the cladding layer.
A further aspect of the invention relates to a method of making the amplifier fiber described above. A preform having a core glass material surrounded by a clad glass material and an overclad glass material is heated. The overclad layer has a refractive index greater than that of the clad or alternatively, the overclad is doped with an absorbing material. The heated preform is drawn under conditions effective to prepare the fiber.
An additional aspect of the invention includes a method of making the amplifier fiber by simultaneously drawing the glass core from a body of molten core glass material, drawing the cladding layer from a body of cladding layer material, and drawing the overclad layer from a body of overclad layer material under conditions effective to form the fiber.
Other aspects of the present invention relate to the amplifier fiber that include the fiber of the present invention and one or more polymeric coating layers which encapsulate the glass fiber; and an optical amplifier or laser that includes the fiber of the present invention, wherein the glass core contains a glass having an effective amount of a dopant which yields a sufficient signal gain over a predetermined length of the optical fiber.
The fiber of the present invention, as well as the amplifiers or lasers formed therefrom, is capable of stripping modes of light that may be propagating along the cladding layer of the fiber. An additional advantage of the invention is that the length of amplifier fiber required to strip the cladding mode can be relatively short. The fiber of the invention also has the advantage that the amount of absorption of the overclad layer may be manipulated to preselected amounts for desired applications of the fiber.
The fiber of the invention also has the advantage of being a low attenuation fiber. Furthermore, the invention has the advantage that it is applicable to glass compositions that were previously unsuitable for use in fiber optics applications.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.